MOLECULAR MECHANICS STUDY OF ZIRCONOCENIUM CATALYZED ISOSPECIFIC POLYMERIZATION OF PROPYLENE

Stereocontrol energy (DELTAE(O)) is investigated as a measure of enant ioselectivity of ansazirconocenium catalyst in propylene polymerizatio n; it was calculated with MM2 (molecular mechanics) force field using pi complex (piC) and transition state (TS) geometries obtained by ab i nitio molecular orbital methods. Both rac-ethylenebis (1-eta5-indenyl) - (1) and rac-ethylenebis (1-eta5-4,5,7,8-tetrahydroindenyl) (2) zirco nocenium species are isospecific in either the pi-complexes or the tra nsition states. The stereoselectivity is greater if there is alpha-ago stic interaction; it is lowered in the cases of beta and gamma agostic interactions. The C-13-NMR steric pentad distribution indicates the p oly (propylene) to be much less stereoregular than that predicted by D ELTAE(O). Following the occurrence of a regiochemical insertion error, the addition of another monomer via any mode is prohibitively unfavor able. The catalyst suffers loss of stereospecificity as temperature of polymerization increases. Insertion via transition states involving d ifferent agostic interactions could be one explanation for the observe d loss. (C) 1995 John Wiley & Sons, Inc.